The invention described herein may be manufactured, used and licensed by or for the Government for governmental purposes without the payment to me of any royalties thereon.
Polarization phase plates made of birefringent crystals have long been useful in the study and various applications of polarized light. However, prior art applications of these crystals, typically in sandwich-like layers, exhibited serious problems such as complex angular dependencies of the through-beam geometry for the induced polarization phase shift, little or no fringe spacing adjustment capability and, most severely, exceedingly poor performance for systems that require a large fringe-count having very fine fringe spacing. Prior art applications typically utilized very thin plates (usually less than 1 mm) having very shallow cut angles (usually less than 10xc2x0). This is due to the fact that, based on certain assumptions that are well-known in the art regarding the e-ray and o-ray components of the incoming light and the relationship between the wave vector and the power flow vector and polarization and phase-shift dynamics at the air-dielectric interface, a fairly valid analysis could be done on the beam output of a thin-plate assembly. But these assumptions do not result in accurate beam analysis for crystals that are thick enough to be practical to fabricate (between about 1 mm and 1 cm thick) and have significant cut angles. A device is desired that uses such thick crystals and can produce a large count, yet high-quality fringes that are straight, smooth and have a sinusoidal cross-fringe profile and as much as 100% contrast when the input beam is fully linearly polarized and oriented for maximum contrast.
Applicant""s invention is a polarization phase plate assembly (hereinafter referred to as xe2x80x9cthe Assemblyxe2x80x9d) comprising at least two uniaxial crystal plates which are identical to each other in structure and dimension and a retarder plate sandwiched between the two crystal plates. Initially the optic axes of the crystal plates are aligned to be parallel with each other. Then the crystal plates are rotated around their normals by an equal twist angle but in opposite directions. When a beam of light is passed through the Assembly, the result is high-quality fringes that vary in number in relation to the degree of the twist angle.